Gas Constants and Physical Constants — Chemistry Reference Values
| Constant | Symbol | Value | Unit | Common Use |
|---|---|---|---|---|
| Gas constant | R | 8.314 | J/(mol·K) | Thermodynamics, SI units |
| Gas constant | R | 8.314 | L·kPa/(mol·K) | Ideal gas law with kPa |
| Gas constant | R | 0.08206 | L·atm/(mol·K) | Ideal gas law with atm (most common) |
| Gas constant | R | 62.36 | L·mmHg/(mol·K) | Ideal gas law with mmHg/torr |
| Gas constant | R | 1.987 | cal/(mol·K) | Thermochemistry in calories |
| Gas constant | R | 0.08314 | L·bar/(mol·K) | Ideal gas law with bar |
| Avogadro's number | Nₐ | 6.022 × 10²³ | mol⁻¹ | Moles to particles conversion |
| Faraday's constant | F | 96485 | C/mol | Electrochemistry, electrolysis |
| Planck's constant | h | 6.626 × 10⁻³⁴ | J·s | Quantum mechanics, photon energy |
| Speed of light | c | 2.998 × 10⁸ | m/s | Electromagnetic radiation, E = mc² |
| Boltzmann constant | kᵇ | 1.381 × 10⁻²³ | J/K | Statistical thermodynamics |
| Elementary charge | e | 1.602 × 10⁻¹⁹ | C | Charge of proton/electron |
| Electron mass | mₑ | 9.109 × 10⁻³¹ | kg | Atomic physics, quantum chemistry |
| Proton mass | mₚ | 1.673 × 10⁻²⁷ | kg | Nuclear chemistry |
| Atomic mass unit | u (amu) | 1.661 × 10⁻²⁷ | kg | Atomic and molecular masses |
| Standard atmosphere | atm | 101325 | Pa | Pressure reference standard |
| Standard temperature | STP | 273.15 | K (0 °C) | Gas calculations at STP |
| Molar volume at STP | Vₘ | 22.414 | L/mol | Ideal gas at 0 °C, 1 atm |
| Molar volume at SATP | Vₘ | 24.790 | L/mol | Ideal gas at 25 °C, 1 bar |
| Rydberg constant | Rₕ | 1.097 × 10⁷ | m⁻¹ | Hydrogen emission spectrum |
| Stefan-Boltzmann constant | σ | 5.670 × 10⁻⁸ | W/(m²·K⁴) | Thermal radiation |
| Coulomb's constant | kₑ | 8.988 × 10⁹ | N·m²/C² | Electrostatic force |
| Water ion product (25 °C) | Kₓ | 1.0 × 10⁻¹⁴ | M² | pH calculations, acid-base |
Values follow CODATA 2018 recommendations. Note that since the 2019 SI redefinition, Nₐ, e, k_B, and h are exact by definition, so R = Nₐ × k_B = 8.31446... J/(mol·K) is also exact. STP as used here is the older IUPAC convention: 273.15 K and 1 atm, giving Vₘ = 22.414 L/mol. The current IUPAC standard is SATP: 298.15 K and 1 bar (not 1 atm), giving Vₘ = 24.790 L/mol. Most US general-chemistry textbooks still use the old STP, so confirm which standard a problem assumes before plugging in. Source: NIST CODATA 2018, IUPAC Gold Book.
Frequently Asked Questions
Which value of R should I use in the ideal gas law?
Match R to your pressure and volume units, and always use Kelvin for T. Pressure in atm and volume in L → R = 0.08206 L·atm/(mol·K). Pressure in kPa, volume in L → R = 8.314 L·kPa/(mol·K). Pressure in mmHg or torr → R = 62.36 L·mmHg/(mol·K). For energy calculations or any work involving thermodynamic potentials, switch to R = 8.314 J/(mol·K). Mixing R = 0.08206 with kPa is the single most common silent error in PV=nRT problems.
What is the difference between STP and SATP?
STP — the older convention — is 0 °C (273.15 K) and 1 atm (101.325 kPa), giving Vₘ = 22.414 L/mol. SATP, the current IUPAC standard since 1982, is 25 °C (298.15 K) and 1 bar (100 kPa, slightly less than 1 atm), giving Vₘ = 24.790 L/mol. The two molar volumes differ by about 10%, so check which standard your problem assumes. US general-chemistry textbooks tend to keep STP; physical chemistry and IUPAC-aligned sources use SATP.
How is Faraday's constant related to other constants?
F = Nₐ × e = (6.022 × 10²³)(1.602 × 10⁻¹⁹ C) = 96,485 C/mol — the charge carried by one mole of electrons. Since the 2019 SI redefinition fixed both Nₐ and e exactly, F is also exact (96485.33212... C/mol). It appears in the Nernst equation E = E° − (RT/nF)lnQ and in Faraday's electrolysis law m = (ItM)/(nF), connecting moles of electrons transferred to coulombs of charge passed through a cell.